1. Field of the Invention
The present invention relates to a color field sequential imaging type electronic endoscope system. More particularly, this invention is concerned with a construction for enabling common use of a light source apparatus for color field sequential illumination irrelevant of a standard television (TV) system of, for example, a display unit.
2. Description of the Related Art
An endoscope, an elongated insertion section of which can be inserted into a body cavity to observe an organ or the like in the cavity or, if necessary, a treatment adaptor inserted through a treatment adaptor channel thereof can be used to conduct various treatments, has been widely utilized in recent years. Such an endoscope is utilized not only in medicine but also in the field of industry.
A variety of electronic endoscopes using charge coupled devices (CCD) or other solid state imaging devices have been proposed and are being put to practical use.
For an endoscope, the diameter of an insertion section must be made small. This restricts the size of a solid state imaging device, making it difficult to increase resolution. In a field sequential method, rays of illumination light with different wavelengths are emitted sequentially via a color rotary filter, and respective color image signals acquired under respective illumination light are synthesized to produce a color image. The field sequential method has the following advantage: the field sequential mode can provide color images with higher resolution than a simultaneous method in which color imaging is performed under white illumination light using an imaging means with color filters.
An example of the foregoing electronic endoscope system of field sequential imaging type has been disclosed in Japanese Patent Laid-Open No.61-82731. A light source apparatus for this electronic endoscope system requires a color rotary filter for sequentially switching white illumination light into red (R), green (G), and blue (B).
On the other hand, a signal processing unit for converting imaging signals acquired by the foregoing field sequential type imaging device into standard TV signals is available in different color television (TV) systems; such as, a system of phase alternation by line (PAL), a system recommended by National Television System Committee (NTSC), and a sequential couleur a memoire (SECAM) system. Accordingly, there exist monitors for these respective systems.
Furthermore, for a light source apparatus, for example, the areas of R, G, and B transmission windows of a rotary filter differ among the foregoing TV systems. The rotational frequency of the rotary filter also differs among the forgoing TV systems. That is to say, control of the rotating speed or the like of the filter differs with the TV system.
Therefore, when an endoscope system has a video processor or monitor of different video or television systems such as PAL and NTSC, light sources for controlling the rotary filter associated with different TV systems such as PAL and NTSC are required, as described above. In other words, multiple kinds of light source apparatuses must be installed.
The present applicant has proposed a light source apparatus for solving the aforesaid problem in Japanese Patent Laid-Open No.1-217413. In this light source apparatus, a rotary color filter for generating color field sequential light rays, formed like a cassette, is detachably provided. In the light source apparatus, the controlled speed of a rotary filter is changed according to the type of an inserted filter cassette.
With the light source apparatus, by changing a rotary color filter cassette and selecting multiple kinds of built-in synchronizing signal generating circuits, this single light source apparatus can cope with multiple TV systems.
Japanese Patent Laid-Open No.3-68330 has proposed a light source apparatus in which rotating speed of a rotary filter and a phasing mode can be changed according to the type of a main unit used in combination with the light source apparatus. According to the electronic endoscope system in this reference, the rotational frequency of the rotary color filter can be changed and servo control with high precision can be achieved. The light source apparatus can also cope with multiple TV systems by varying rotational frequency of the rotary color filter.
As described above, a field sequential type electronic endoscope system employs a color field sequential illumination light source apparatus having a repetition period that is dependent on a TV system of a display unit, because the frame frequency of an image signal differs with the TV system of a display unit. The NTSC system, for example, employs a light source apparatus that emits RGB illumination light at a repetition period of 29.97 Hz, while the PAL system employs a light source apparatus having a repetition period of 25 Hz. In these systems, imaging in each of R, G, B colors is completed in units of a one-frame period of display. Therefore, writing or reading in or from memories, which is intended to synchronize color field sequential signals, can be controlled easily. However, since the aforesaid conventional light source apparatus includes multiple servo circuits internally, the circuitry becomes large in size and complex, and the number of control processes increases. This is a primary reason for a high cost.
Alternatively, different video processors are installed depending on multiple TV systems of a monitor and a recording unit, as described above. Consequently, light sources must be developed independently for the processors. This deteriorates development efficiency. Moreover, mass production effect cannot be expected because a light source apparatus cannot be used in common.